1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
A motor drive circuit is known that is configured to control ON/OFF of a transistor connected to a motor coil so that a current flowing through the motor coil is kept constant (see, e.g., Japanese Laid-Open Patent Publication No. 2008-141831).
FIG. 7 is a diagram illustrating an example of a motor drive circuit 200 configured to drive a motor coil L with a constant current. A control circuit 300 controls ON/OFF of PMOS transistors 310 and 311 and NMOS transistors 312 and 313 based on an oscillation signal Vosc with a predetermined period and a voltage Vr corresponding to a current IL. For example, as indicated at time t0 of FIG. 8, when the oscillation signal Vosc goes high (high level), the control circuit 300 turns on the PMOS transistor 310 and the NMOS transistor 313. In this case, since the control circuit 300 turns off the PMOS transistor 311 and the NMOS transistor 312, the current IL increases while flowing through a path A1.
When the current value of the current IL reaches a predetermined value at time t1, the control circuit 300 controls an H-bridge circuit 301 so as to reduce the current IL, and thereafter results in turning off all the transistors of the H-bridge circuit 301.
As a result, the current IL decreases while flowing through a path A2 via diodes 320 and 321. At time t2, the control circuit 300 turns on again the PMOS transistor 310 and the NMOS transistor 313. Since such an operation is repeated, the motor coil L is driven with a constant current.
When the NMOS transistor 313 is turned on at time t0 and time t2, a reverse recovery current from the diode 321 flows to the NMOS transistor 313. Therefore, in general, radiation noise caused by the reverse recovery current is generated with the timing when the NMOS transistor 313 is turned on. The period of radiation noise generation, which is caused by the reverse recovery current, is identical to the period of the oscillation signal Vosc, i.e., the chopping period of the motor drive circuit 200. Therefore, for example, when a plurality of motor coils are driven with a constant current, if the chopping period varies with the motor coils, the radiation noises are generated with various periods. In such a case, the influence of the radiation noises increases, which leads a user to the needs for taking measures for every radiation noises generated with various periods.